The present invention relates to a method of and an arrangement for contactless rotary speed measurements.
More particularly, it relates to a method of measurements of the rotary speed of a rotating part, in particular a part which is surrounded by a housing.
For determination of a rotary speed of a rotating part usually markings are applied on the rotating part and then sensed by a special sensor, for example an inductive sensor. When the rotating parts are completely surrounded by a housing, such a rotary speed determination is no longer possible. In electric fuel pumps, in particular, which are completely encapsuled, there are no rotating parts which extend outwardly so that other rotary speed measurements are required.
German document DE-P 41 33 269 which corresponds to U.S. patent application Ser. No. 070,425, now U.S. Pat. No. 5,424,636 discloses a rotary speed determination in accordance with which, and especially in connection with a measurement of a rotary speed of the electric fuel pump, the current signal of the electric fuel pump is evaluated. By alternating current supply of the armature winding during the rotation of the armature, rotary speed dependent current fluctuations are produced. These current fluctuations which occur with a rotary speed-dependent frequency are digitalized by sensing the current signal and twice fourier transformed in a computer device. After the second fourier transformation a spectrum is obtained in which the rotary speed is determined directly from the absolute maximum.